While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using a KrF, ArF or F2 laser as the light source is strongly desired to reach the practical level as the micropatterning technique capable of achieving a feature size of 0.3 μm or less. Various alkali-soluble resins are used as the base resin in such resists.
For KrF laser resists, a polyhydroxystyrene resin having phenolic hydroxyl groups as the alkali-soluble functional group is, in fact, a standard base resin. For ArF laser resists, poly(meth)acrylate resins using carboxyl groups as the alkali-soluble group and resins comprising polymerized units of cycloaliphatic olefin such as norbornene are under investigation. Of these, the poly(meth)acrylate resins are regarded, due to ease of polymerization, as a promising candidate that will find practical use. For these resist resins using as the alkali-soluble functional group carboxyl groups having a higher acidity than phenolic hydroxyl groups, however, an outstanding issue is difficulty of dissolution control, often leading to pattern collapse caused by swelling or the like.
Functional groups having an acidity comparable to phenolic hydroxyl groups are desired. It was proposed to use an alcohol having a plurality of fluorine atoms substituted at α- and α′-positions (e.g., having a partial structure: —C(CF3)2OH) as the alkali-soluble functional group, as described in G. Wallraff et al., Active Fluororesists for 157 nm lithography in 2nd International Symposium on 157 nm Lithography. Styrene and norbornene derivatives having fluoroalcohol —C(CF3)2OH incorporated therein are proposed as monomers used in the manufacture of base resins. Similar examples of fluoroalcohol-substituted norbornene are found in JP-A 2003-192729 and JP-A 2002-72484. For the polymerization of norbornene monomers, however, radical polymerization of monomers of the same type is difficult, and instead, special polymerization techniques such as coordinate polymerization using unique transition metal catalysts and ring-opening metathesis polymerization are necessary. Although alternating copolymerization between a norbornene monomer and a comonomer such as maleic anhydride or maleimide can be implemented by radical polymerization, the presence of comonomer imposes a substantial limit on the freedom of resin design.
JP-A 2003-040840 describes fluoroalcohol-substituted acrylate monomers. Although the method of preparing these monomers is not definite, the starting reactant used is hexafluoroacetone (boiling point −27° C.) which is awkward to handle because it is gaseous at room temperature. The synthesis of polymerizable compound must follow many steps, leaving the problems of an increased cost and difficult commercial implementation.
There is a strong demand to develop a polymerizable compound (or monomer) having both a polymerizable unsaturated group such as a (meth)acrylate structure and a functional group having an acidity comparable to phenolic hydroxyl, which compound can be prepared and polymerized both in an industrially acceptable and economic manner.
Over a decade, photolithography using ArF excimer laser light (193 nm) has been under active investigation. It was expected at the initial that the ArF lithography would be applied to the fabrication of 180-nm node devices. However, the KrF excimer lithography survived to the mass-scale fabrication of 130-nm node devices. So, the full application of ArF lithography started from the 90-nm node. The ArF lithography combined with a lens having an increased numerical aperture (NA) of 0.9 is considered to comply with 65-nm node devices. For the next 45-nm node devices which required an advancement to reduce the wavelength of exposure light, the F2 lithography of 157 nm wavelength became a candidate. However, for the reasons that the projection lens uses a large amount of expensive CaF2 single crystal, the scanner thus becomes expensive, hard pellicles are introduced due to the extremely low durability of soft pellicles, the optical system must be accordingly altered, and the etch resistance of resist is low; the postponement of F2 lithography and the early introduction of ArF immersion lithography were advocated (see Proc. SPIE Vol. 4690 xxix).
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens with NA of 1.0 or greater. Theoretically, it is possible to increase the NA to 1.44. The resolution is improved by an increment of NA. A combination of a lens having NA of at least 1.2 with ultra-high resolution technology suggests a way to the 45-nm node (see Proc. SPIE Vol. 5040, p 724).
Several problems associated with the presence of water on resist were pointed out. For example, projection lens contamination and pattern profile changes occur because the acid once generated from a photoacid generator and the amine compound added to the resist as a quencher can be dissolved in water. Inversely, swelling and circular defects known as water marks occur because water can penetrate into the resist film. For overcoming these problems, it was proposed to provide a protective coating between the resist and water (see the 2nd Immersion Workshop, Jul. 11, 2003, Resist and Cover Material Investigation for Immersion Lithography); and to prevent resist materials from dissolution in water or water penetration by controlling the water repellency of resist materials, typically photoacid generators (PAG) or base resins (see J. Photopolymer Sci. and Technol., Vol. 18, No. 5, p 603 (2005)).